There is an item I'd like to adjust.
What this does is increase your effective Druid level by four just for purposes Wild Shape (fairly obvious). What I would like to do is create a similar item for Animal Companion. The result would be:
"Headband of the Animal Companion [40,000 GP, Head]"
This would increase effective druid level by four for Animal Companion.
Would this be allowed?

At work now, please write down what the item does. If you want to make an item of the some type for your animal companion the SRD has prices listed for changing size and shapes of magical items. However I will say magical items for this campaign shape to the size of the wielder of the item after 8 hours of attunement. For example a giant who found a magic ring on a dead halfling could wear the ring, and after 8 hours the rings abilities could be discovered by the giant.

My app is pretty much ready for review, I still have a little more editing and the castle to flesh out, had an emergency at work I had to fix and it took me 4 hours. I will try to map out the layout for u if you want to use it. And there will be machiculations!!!!!!

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Some may question your right to destroy ten billion people. Those who understand realize that you have no right to let them live!

Worn in the headslot by a character with an animal companion (not the companion).

While wearing this Headband, for purposes of the Animal Companion ability the wearer is treated as if they are four levels higher in their base class that provides this ability (Ranger, Druid, etc.). This allows you to progress or acquire a single companion accordingly as if you were that level. This does not affect your ability to command your companion, use wild empathy, or to train your companion. Once this headband is used with a specific companion, it cannot be used with another companion. If the wearer wishes to acquire a new companion, they must obtain a new headband.

My app is pretty much ready for review, I still have a little more editing and the castle to flesh out, had an emergency at work I had to fix and it took me 4 hours. I will try to map out the layout for u if you want to use it. And there will be machiculations!!!!!!

Yes. I wouldn't usually do this, but this headband is actually quite powerful as it provides an effect literally no other magic item in the game does. Additionally, it's impact is primarily at the current level. Within four levels I'll be at the non-epic max tier for animal companion, and by that point the difference in creatures between one tier and another drops off (largely because your power is centered around wild shaping and casting, not the companion).

Animal companions are best used as tactical beatsticks/meatshields, with a few auxiliary uses revolving around a few interesting collars (Beastfriend collar that allows a really weird form of unlimited blinking, kinda) or feats (Exalted Companion on a druid allows your animal to take the Vow of Poverty, oddly enough, which has it's own interesting implications). Some more odd companions work with debuffs, such as the sailsnake which has a guaranteed (saving reduces the duration but doesn't prevent) blinding effect for all enemies in a large cone, and can cycle that attack several times, and stack it with venomfire for high damage. Watchspiders are the only intelligent magic beasts allowed as animal companions, which has its own set of cheese. Overall, however, it's still revolving around getting into combat, staying in combat, winning combat, ending combat. The bigger companions also help with transport usually (Rocs are my favorite on that note). The big problem, overall, is that you can't get anything but animals/dinosaurs. At best you get Celestial, Warbeast, or Horrid templates (or, in the case of two specific exceptions to an overall rule, Magebred templates) on there, none of which offer the utility that would value a companion out of combat.

This is great, but once you hit epic levels if your playing a druid right you are so insanely capable just eliminating everything as a Druid yourself that the need for companions slacks off. Going insanely hard into this, I believe there some PrCs that do make it worth it, but just barely. It would make more sense to progress wild shaping than it would a companion, because the insanity that PlanShep allows is worth doing 50 levels in all druid-based classes only.

Animal Companion, as a class ability, has the potential for awesomeness. At levels 1-10, it is insanely OP. At lvls 10-20, it slowly falls off in power but is still quite good on many counts. Once you wade into epic levels, though, it just becomes redundant. You can get far better creatures as summons, and your summon durations will be more than long enough to last through both fights and utility uses. The reason animal companion is a rather stunted ability long-term is that it has a glaring absence of critical feats. For example, Wild Shape has Aberration Shape, Dragon Shape, Frozen Shape, Exalted Shape, and far more...in ADDITION to getting animals, plants, and elementals/paraelementals just from the class. Animal Companion? You can get Exalted, which doesn't do much because Celestial sucks as a template. In other words, Animal Companion drastically lacks type-adding feats that could expand your list to be viable end game. (DragMag does have some really, REALLY weird options out there, though, but getting the actual reference material is usually a pain so I don't count those unless I have the full citation usually).

There is an ACF that allows you to get elemental companions, but the problem there is that most elementals really aren't more useful than an animal of equal HD, especially using Eberron Gatekeeper Druids that get Horrid animals. Even worse, there is no provision for treating these as a true animal, thus making 99% of your companion-buffing spells useless because they can't be cast on an elemental.

Even worse (if that's possible), most animal companion oriented PrCs don't solve this problem. They either give variety without much improvement, or they focus intently on a single creature...that isn't any more noteworthy than a top-tier companion like a Roc or Horrid Elephant. A few have progressions that could shove a companion into relevance for a few more levels, perhaps, but the sacrifices the classes take to get there aren't worth what you get out of it. (I'm looking at you, Arcane Hierophant).

Overall, since this is a homebrew campaign, once we hit epic levels I might try to get a feat or two approved that allows for alternate companions, but also allows you to treat them as animals for the sake of your own abilities and spells. This would make the companion actually relevant end-game (unlike now).